The subject of the present invention is a coating composition for a metal substrate, based on an aqueous sodium and/or potassium and/or lithium silicate solution.
The present invention also relates to the methods of applying this composition to the said metal substrate and to the various uses of this coating composition.
According to the present invention, the coating composition is intended to be applied to a steel substrate having a free outer surface formed by a zinc or zinc-based alloy metal layer.
In one advantageous embodiment, the said metal layer may have been deposited on the said steel substrate electrolytically or by hot dipping.
As an example of metal substrates, mention may be made of:                electrogalvanized sheets: steel substrate coated with a zinc layer applied electrolytically;        hot-dipped galvanized sheets: steel substrate coated with a zinc layer applied by dipping the said sheet into a bath of molten zinc;        GALFAN: steel substrate coated with a layer of a zinc (95 wt %)/aluminium (5 wt %) alloy applied by dipping in a bath of molten zinc/aluminium alloy having the same proportions; and        GALVALUME®: steel substrate coated with a layer of an aluminium (55 wt %)/zinc (45 wt %) alloy applied by dipping in a bath of molten aluminium/zinc alloy having the same proportions.        
The treatment of a metal surface is subject to many simultaneous constraints of a technical, economic and environmental type.
Coils of sheet are produced in steel plants by very rapid processes, the line speed of which may range from a few m/min up to 250 m/min. When it is desired to couple the production process to a surface treatment step, the surface treatment technologies must comply with these line speed constraints. Technical difficulties may therefore arise if it is desired to maintain good chemical reactivity between the substrate and the treatment products, but also as regards film formation if it is desired to obtain good flow and good homogeneity of the film coatings.
Anticorrosion treatment technologies usually make use of chromium-based products (the chromium being hexavalent or trivalent) that are applied as one or more layers. However, these products are environmentally harmful and have to be replaced with treatment products having no impact on the environment.
In addition, manufacturers at the present time are seeking to employ a technology that meets a minimum specification and is capable of being functionalized so as to meet stricter requirement levels and to increase the added value of the surface treatment.
As a complement to corrosion resistance, the functionalization of the treatments relates in particular to the following fields:                the surface properties such as the hydrophobicity or hydrophilicity of the surface, the anti-fingerprint character of the surface or else the modification of the free alkalinity of the surface;        the application properties, such as the flexibility and the lubrication of the coating for bending or drawing operations, but also the electrical conductivity in the case of joining by welding.        
Manufacturers are also subject to economic constraints and thus seek compact treatments using formulations that are, on the one hand, prepared in aqueous medium (and that therefore do not require additional investment for the reprocessing of possible organic solvents) and that, on the other hand, allow application of a single layer with a single drying operation.
Finally, the on-line operating constraints also require products that are, on the one hand, one-component products (i.e. products that do not require the preparation of a mixture of several products prior to industrial application) and are, on the other hand, stable over time (i.e. have a lifetime of more than three weeks in order to accommodate campaign production runs).